The present invention relates generally to control of a power transmission in a work vehicle such as an agricultural tractor. More particularly, the invention relates to an improved technique for shuttle shifting such transmissions based, in part, on the vehicle speed and the selected gear to which a shift is to be made.
In the field of transmission systems for work vehicles, such as agricultural tractors, a number of transmission configurations and control schemes have been proposed and are presently in use. Such transmissions typically include a collection of intermeshing gears either fixed to transmission shafts or rotating freely on the shafts. Clutches associated with the freely rotating gears may be selectively engaged to establish a series of speed ratios between an engine output shaft and a transmission output shaft to transmit engine torque at a desired speed to driven wheels of the vehicle. Control systems for commanding engagement of the clutches typically include electronic circuitry that responds to operator controls, such as an upshift/downshift lever, a forward/reverse lever and the like in the vehicle cab. The control system sends electric signals to hydraulic valves that channel pressurized fluid to the clutches. The control systems thus cause the clutches to engage and disengage in predetermined combinations to accelerate, decelerate and drive the vehicle as desired by the operator. Transmissions and control systems of this type are described in U.S. Pat. No. 4,425,620, issued on Jan. 10, 1984 and assigned to Steiger Tractor, Inc., and U.S. Pat. No. 4,967,385, issued on Oct. 30, 1990 and assigned to J. I. Case Company.
Control systems of the type described above have been proposed that permit an operator to command direct shifting between forward and reverse gear ratios by movement of a gear shift lever or similar command device. In general, such systems disengage the clutches corresponding to the then current gear ratio and engage, typically through a fluid modulation scheme, clutches corresponding to the target gear ratio. A system of this type is described in U.S. Pat. No. 4,967,385 mentioned above. Variations on this general approach to shifting have been proposed that permit controlled deceleration on the vehicle during shuttle shifts by completely disengaging and stopping the transmission, then re-engaging the transmission to decelerate the vehicle in preparation for engagement of the selected gear ratio. A system generally of this type is described in U.S. Pat. No. 4,989,470, issued on Feb. 5, 1991.
While such systems have provided a degree of convenience, particularly for driving vehicles through repetitive changes in direction, they are not without drawbacks. For example, systems for controlled deceleration during shuttle shifts can be complicated to implement and include deceleration features which may be superfluous, considering that common shuttle shifting situations are usually encountered when vehicle speed is already relatively low. Moreover, known control systems for shuttle shifting power shift transmissions typically employ that same control schemes regardless of the target gears to be ultimately engaged following the shuttle shift. While this is typically acceptable where the target gear ratio provides relatively low vehicle speed in the reverse direction, directly engaging a reverse direction gear providing high vehicle speed may result in unnecessary strain or heating of the transmission elements. In addition, known shuttle shifting control systems typically do not include satisfactory schemes for re-engaging the vehicle transmission in the event of an aborted shuttle shift.
There is a need, therefore, for an improved control system for permitting and controlling shuttle shifting operation of power shift transmissions of the type found in work vehicles, such as agricultural tractors. In particular, there is a need for a control system that addressed the drawbacks of existing systems, affording convenient shifting of a transmission between forward and reverse gear ratios, taking into account the ultimate gear ratio or vehicle speed to be attained in the shift. There is also a need for a control system of this type designed to respond to overspeed shuttle shifts and aborted shuttle shifts in a convenient and intuitive manner from operator's standpoint.